The present invention relates to a pre-winding camera of the type adapted to pre-wind all frames of a filmstrip loaded therein before exposures and to rewind it one frame for every exposure and more particularly to a prewinding control device for the pre-winding camera.
It is known to provide an electrically powered film driving device for winding all frames of a filmstrip before making exposures in modern cameras, especially 35mm cameras. Such a camera, which is called a pre-winding camera, is provided with electrically powered film drive means such as an electrically powered reverse-drive motor that is interchangeably operationally coupled to a film take-up spool in a film chamber in order to withdraw a filmstrip from a patrone or cartridge and wind it up onto the take-up spool before making exposures or just after loading of the patrone and in order to rewind the exposed filmstrip onto a spool disposed in the patrone, one frame of the filmstrip for every exposure.
As is well known, the pre-winding camera is adapted to start the film drive motor in response to closing a back door of the camera for winding up all frames of the filmstrip onto the take-up spool in the film chamber. When all frames of the filmstrip are wound onto the take-up spool, the film drive motor is interrupted in rotation and terminates prewinding of the filmstrip. More specifically, there is provided in such a pre-winding camera sensing means for sensing detectable means, such as sprocket holes or perforations, formed along a filmstrip and means for generating signals, one for every predetermined number of the detectable means detected by the sensing means. For example, when a sprocket wheel engaged with the sprocket holes or perforations in the filmstrip turns one revolution for every advancement of a predetermined length of filmstrip, the pulse signal generating means provides one pulse signal. Since the trailing end of the filmstrip is secured to a spool of the patrone, when all frames of the filmstrip are wound onto the take-up spool, the filmstrip cannot be withdrawn from the patrone any more, interrupting the rotation of the film drive motor. Consequently, film advancement is terminated and the pulse signal generating means stops generating pulse signals. Therefore, when no pulse signal is provided for a predetermined period of time, it is decided that the filmstrip has been fully would up. Upon this decision, the film drive motor is stopped. This predetermined period of time (which is hereinafter referred to as an interruption time interval) is set to be relatively long so as not to determine the completion of pre-winding of the filmstrip when a time interval at which pulse signals are generated becomes longer due to a drop of battery power.
One problem in such a pre-winding camera is that the film drive motor is left interrupted in rotation for a time interval longer than the interruption time interval independently of the battery power. In order to prevent the film drive motor from generating heat by interruption in rotation and from breaking down, it is desirable to shorten the interruption time interval in the case where an electric current flowing to the film drive motor is or becomes large due to the interrupted rotation of the film drive motor such as, in particular, when the battery is fresh and, therefore, has a high power. On the other hand, if the interruption time interval is too short, it is apt to make a wrong decision when pulse signals are generated at prolonged pulse generating time intervals, in particular when the filmstrip is gradually withdrawn out of the patrone at the beginning of withdrawal due to a curled film leading end or to friction caused between the film strip and the film patrone. For better protection of the electric film drive motor when batteries for the film drive motor are still fresh, it is unfavorable to shorten the interruption time interval.